It should be interpreted that you are unable to gain the Spell in the first place as they are not treated like Objects.
The rules state on page 13 - "All characters may acquire and cast Spells, if their Craft is sufficient to permit this."
So if you Craft is not sufficiently high, or you are already carrying your full complement of Spells, you may not acquire another.
"Any character acquiring more than four Objects must decide which to keep. The remainder are immediately placed faceup on the character’s space."
This applies to when you find or take an Object and make a choice as to which to carry with you.
"If at any time a character has more Spells than his Craft allows, surplus Spells must immediately be placed on the Spell Card discard pile; they cannot be cast."
This only applies to if your Craft were to drop for some reason.
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